This invention relates to piezoelectric transformers generally in the shape of a ring.
Piezoelectric transformers are devices that step-up or step-down AC voltage or current by piezoelectric vibration. Such transformers have several advantages over electromagnetic tranformers. First, it has a lager power to volume ratio such that it is easier to be miniaturised. Second, piezoelectric transformers use electromagnetic-noise free as piezoelectric vibration is involved instead of an electromagnetic field. Further, since such transformers have no windings the hazard of overheating and/or fire may be substantially reduced.
In general, piezoelectric transformers may be classified into two types. One type is being used as step-up transformers in which the voltage gain is larger than one. Another type is used as step-down transformers with voltage gain of less than one. A single plate structure is usually adopted in the step-up piezoelectric transformers, while a multi-layered structure is usually adopted in the step down transformers.
In U.S. Pat. No. 2,974,296, a variety of piezoelectric transformers having a general shape of a disc were disclosed. FIG. 1 is a reproduction of an embodiment shown in U.S. Pat. No. ""296. In this particular embodiment, input signal is supplied to electrode 1 and a common electrode on the bottom surface (not shown in the figure), while the output signal is taken between electrode 3 and the common electrode.
FIG. 2 shows yet another version of a piezoelectric transformer described in U.S. Pat. No. ""296. In this particular embodiment, the piezoelectric body 4 is divided into two semi-circular regions. In driving region 5, the piezoelectric body 4 is polarised across the thickness of the body. Accordingly, the output region 6 is polarised in opposing tangential directions as shown in FIG. 2. When an input signal is supplied to the input electrode associated with the output region 5, vibration results in the change of radius and circumference of the piezoelectric body 4. The variation in circumference results in strains and corresponding output signals in output region 6.
The piezoelectric transformers as described in U.S. Pat. No. ""296 having a general shape of a disc way have the drawback of being relatively difficult to manufacture as the piezoelectric body includes several portions which are required to be polarized differently.
U.S. Pat. Nos. 5,814,922 and 6,040,654 describe piezoelectric transformers having a general ring-shaped piezoelectric body. One possible disadvantage associated with these transformers may be that they are relatively difficult to fabricate due to their complex structures. In U.S. Pat. No. 5,814,922, a piezoelectric transformer with its body being divided into at least eight sections is described. In U.S. Pat. No. 6,040,654, a piezoelectric transformer having at least eight different sections is described, with each section being connected by relatively delicate connection rods. The difficulty to manufacture may become more apparent if their size is substantially reduced.
Therefore, it is an object of this invention to provide a piezoelectric transformer having a relatively simple structure that may be fabricated easily. It is yet another object of this invention to provide a piezoelectric transformer that may be miniaturised readily.
As a minimum, it is an object of the present invention to provide the public with a useful choice.
Accordingly, this invention provides a piezoelectric transformer including a substantially ring-shaped piezoelectric material having first and second surfaces separated by a thickness dimension defining a thickness direction. The first surface is at least partially covered by a common electrode, while, the second surface includes first and second segments each at least partially covered by corresponding first and second electrodes. The first and second electrodes are separated electrically by a substantially ring-shaped separating section. Under such a construction, when an input signal is applied to the first electrode, a vibration is excited along a radial direction that is substantially perpendicular to the thickness direction, and a stress is developed across the radial direction, resulting in an output signal generated at the second electrode.
Additionally, the separating section is located at a position where the stress is zero. More preferably, the separating section has a radius substantially equal to the average value of the inner and outer radii of the piezoelectric material.
Optionally the first and second electrodes are in contact with conducting elements for applying or sensing signals to the first and second electrodes, that the conducting elements are in contact at nodal points of the vibration. More preferably, the nodal points of displacement of the first and second segments are substantially in the middle of the corresponding first and second segments.
Preferably, the piezoelectric transformer operates in third symmetric extensional vibration mode having a resonance frequency. More preferably, the input signal has a frequency substantially equals to the resonance frequency of the third symmetric extensional vibration mode.
Optionally, the piezoelectric transformer may operate in the either one of fourth to seventh symmetric extensional vibration mode.
Preferably, the first and second segments each are fully covered by corresponding first and second electrodes.
It is another aspect of this invention to provide a piezoelectric transformer including a substantially ring-shaped piezoelectric material having first and second surfaces separated by a thickness dimension defining a thickness direction, wherein
the first surface is at least partially covered by a common electrode;
the second surface includes first and second segments each at least partially covered by corresponding first and second electrodes, such that when an input signal is applied to the first electrode, a vibration is excited along a radial direction that is perpendicular to the thickness direction, and a stress is developed across the radial direction, resulting in an output signal generated at the second electrode; and
a substantially ring-shaped separating section electrically separating the first and second electrodes locating at a position where the stress is zero.
It is a further aspect of this invention to provide a piezoelectric transformer including a substantially ring-shaped piezoelectric material having first and second surfaces separated by a thickness dimension defining a thickness direction, wherein
the first surface is at least partially covered by a common electrode;
the second surface includes first and second segments each at least partially covered by corresponding first and second electrodes, such that when an input signal is applied to the first electrode, a vibration is excited along a radial direction that is perpendicular to the thickness direction, and a stress is developed across the radial direction, resulting in an output signal generated at the second electrode; and
a substantially ring-shaped separating section electrically separating the first and second electrodes
wherein the first and second electrodes are in contact with conducting elements for applying or sensing signals to the first and second electrodes, such that the conducting elements are in contact at nodal points of the vibration.
It is yet another aspect of this invention to provide a system for generating an output signal from an input signal including at least two piezoelectric transformers connected in parallel, wherein each piezoelectric transformer includes a substantially ring-shaped piezoelectric material having a first and second surfaces separated by a thickness dimension defining a thickness direction, wherein
the first is at least partially covered by a common electrode;
the second surface includes first and second segments each at least partially covered by corresponding first and second electrodes; and
a substantially ring-shaped separating section electrically separating the first and second electrodes
such that when an input signal is applied to the first electrode, a vibration is excited along a radial direction that is perpendicular to the thickness direction, and a stress is developed across the radial direction, resulting in an output signal generated at the second electrode.